It is conventional practice when drilling through underground rock to drill around a central cylinder of rock which is subsequently detached and brought to the surface as a sample, habitually referred to as a rock core. Once brought to the surface, rock cores may be subjected to various measurements and tests.
One known test procedure entails centrifuging the sample at various speeds and measuring the amount of liquid which drains from the sample at the various speeds. This enables determination of a capillary pressure curve as described in Hassler and Brunner “Measurement of Capillary Pressure in Small Core Samples”, Trans. AIME, vol 160, pages 114-123 (1945) also published as Society of Petroleum Engineers paper SPE 945114-G. This method is also discussed in Chen and Balcom, Journal of Chemical Physics vol 122, 214720 (2005) and the related U.S. Pat. No. 7,352,179 (Chen and Balcom, Green Imaging Technologies). The teaching of Chen and Balcom develops the method further by proposing that the amount of fluid in the porous sample is determined by nuclear magnetic resonance (NMR). After centrifuging for a period of time, the sample is removed from the centrifuge and placed in a nuclear magnetic resonance spectrometer. A quantitative method for determining the spatial distribution of fluid used after transfer to the NMR spectrometer is the “SPRITE” technique described in Balcom et al “Single Point Ramped Imaging with T1 Enhancement (SPRITE)”, J. Magn. Reson. A vol 123, pages 131-134 (1996).
U.S. Pat. No. 7,352,179 envisages that the centrifuge and the NMR spectrometer may be separate pieces of apparatus. The sample will be transferred from the former to the latter after centrifuging for a period of time. The document recognizes that “Centrifugation creates a non-equilibrium fluid distribution which will change due to capillarity, diffusion and inlet/outlet drying once the sample is removed from the centrifuge”. It is suggested that these changes may be measured by NMR after centrifugation.
FIG. 20 of U.S. Pat. No. 7,352,179 shows an alternative arrangement in which a disc shaped sample is centrifuged by spinning around the disc axis while located inside the radio frequency coil and magnetic field of an NMR spectrometer. The disc axis is transverse to the magnetic field direction, so that the disc spins in a plane parallel to the field direction. U.S. Pat. No. 7,352,179 states that the amount of fluid within the spinning rock disc can be monitored by “NMR bulk measurements, such as free induction decay and CPMG (Carr-Purcell-Meiboom-Gill)”. After centrifuging for a period of time, the spin is ceased and a 2D magnetic resonance imaging method is used to obtain the 2D spatial distribution of fluid in the plane of the disc and from this to assess saturation radially relative to the disc axis.